The present invention relates to the production of Xenon for use in magnetic resonance imaging, and more specifically for the bulk production and usage of hyperpolarized .sup.129 Xenon.
Highly spin-polarized .sup.129 Xenon nuclei and .sup.3 Helium nuclei offer prospects of greatly improved medical magnetic resonance imaging (MRI) as more fully discussed in "General Review Article : Physics Today, June 1995, p. 17-18". It is known that the MRI contrast available with hyper-polarized .sup.129 Xenon is enhanced by 5 orders of magnitude compared with the contrast obtainable with ordinary equilibrium polarizations at room temperature, resulting in images of the lung cavity comparable to, or better than, those seen from protons in dense tissues. Furthermore, by exchanging polarization with protons in tissue or blood, the polarization of the latter can be enhanced, in some situations so as to increase greatly the sensitivity and resolution of MRI images, which could revolutionize MRI imaging techniques and open up new diagnostic applications. The exploratory medical applications thus far have been made with xenon polarized by the optical pumping technique more fully described in "Optical Pumping Method": W. Harper et al., Phys. Rev. A29, 3092 (1984), which can achieve suitably high polarizations, but because polarization is produced in the gaseous phase, it is costly and not very suitable for the production, storage and distribution of large quantities, which are almost certainly going to be demanded.
The general nature of the polarization methods of the present invention are related to concepts which have been developed for polarized H and D nuclei in HD molecules, for the purpose of producing polarized H and polarized D for nuclear and particle physics targets, and for other uses. U.S. Pat. No. 4,642,206 teaches these polarization processes applied to HD, but the principles rather than the particular material are the basis of the present invention and these apply to xenon and to other nuclei. In these HD and in the hyper-polarized xenon materials, a necessity is to utilize very low temperatures and high magnetic fields and spin-polarize large amounts of material in the condensed phase. The apparatus for obtaining large polarizations in high magnetic fields is cumbersome, and the main principle is to polarize under conditions wherein the relaxation times are reasonably short, for example of the order of a day, and then effect a great increase in the relaxation times so that the material can be considerably warmed, and with only a modest surrounding magnetic field, still retain for long times the polarization earlier achieved. This core concept is the "relaxation switch", manifestations of which are described in published journal articles: "HD Polarized Targets": A. Honig, Q. Fan, X Wei, A. M. Sandorfi and C. S. Whisnant, Nuc. Instrum. and Methods A356, 39(1995) and "HD Polarized Targets": A. Honig et al., 12.sup.th Intl Symposium on High Energy Spin Physics, Amsterdam, Sep. 10-14, 1996. p. 365. World Scientific, 1997, and in the U.S. Pat. No. 4,642,206 patent referred to above. For the HD, one form of relaxation switch described involves introducing a concentration of H.sub.2 and/or D.sub.2 impurities in a metastable state in which the molecules have rotational angular momentum (ortho-H.sub.2 and/or para-D.sub.2). These impurities permit rapid relaxation to occur among the host HD molecules. One then waits for conversion of these impurities to a benign, rotationless state, thus shutting off the relaxation and allowing the polarization to remain for long times even at ordinary liquid helium temperatures and at modest fields less than a Tesla, in a so-called "frozen-spin" state. In this case, the "frozen-spin" occurs in a state more accessible for usage (higher T, lower B) than the state in which the polarization is originally produced (lower T, higher B). This is unlike many other frozen-spin processes, particularly those involving electron-nuclear dynamic polarization, which are discussed in the literature "12.sup.th Intl. Symposium on High Energy Spin Physics, Amsterdam, Sep. 10-14, 1996. p. 365. World Scientific, 1997", where the usage temperature is often lower than that used in the polarization process. This relaxation switch process is the basis of large polarized nuclear HD targets which are now in the process of being prepared for usage in nuclear physics laboratories and national accelerator sites, i.e., "LEGS Project at Brookhaven National Laboratory, Upton, N.Y.; GRAAL Project at Grenoble, France". The polarized D in HD also has application to nuclear fusion fuels, as do polarized tritium nuclei and .sup.3 He nuclei, for which polarization techniques have been put forward. Another relaxation switch proposed for the HD in the aforementioned U.S. Pat. No. 4,642,206 patent involves introduction of radicals by irradiation or other means, which catalyze the relaxation at the polarizing conditions of very low temperature and high magnetic field. The relaxation switch-off is then achieved by inducing recombination by waiting or annealing at higher temperatures, or by quick conversion to a liquid phase, carried out in a time sufficiently short so that the polarization doesn't decay before refreezing is effected. Other relaxation switches conceived of for the `hydrogens` molecules include turning off the relaxation due to impurities by effecting a change of phase, such as in D.sub.2, where the relaxation time in the liquid phase exceeds that in the solid phase near the melting region.
It is therefore an object of this invention to provide methods of polarizing solid xenon which can result in large quantities of the hyper-polarized xenon at a lower cost and in a more favorable form than that envisaged by the conventional optical pumping method.
The present invention is directed towards a polarization process for producing bulk hyper-polarized solid .sup.129 Xenon in a fairly short time (less than 3 days), and several relaxation switches particularly relevant to spin-polarized .sup.129 Xe, allowing its use for enhanced magnetic resonance imaging (MRI) for medical and biological research applications, and for other applications as well in the gaseous phase and "frozen-spin" configuration. We also describe several configurations of the apparatus for polarizing and utilizing hyper-polarized .sup.129 Xenon.